Hydraulic actuator apparatus are known for operating a mechanism at a remote location by way of a master cylinder connected to a slave cylinder installed at the remote location. A conduit interconnects the master cylinder to the slave cylinder and the hydraulic apparatus is filled with hydraulic fluid such that when the piston of the master cylinder is actuated, the piston of the slave cylinder, and consequently the piston rod or output member, are simultaneously actuated by displacement of hydraulic fluid from the master cylinder to the slave cylinder through the conduit.
More particularly, it is known to provide such hydraulic apparatus for operating the release mechanism of a friction clutch of a motor vehicle. As disclosed in Letters U.S. Pat. Nos. 4,407,125, 4,503,678 and 4,506,507, all assigned to the assignee of the present invention, it is further known to preassemble friction clutch hydraulic release apparatus by filling the master cylinder, the slave cylinder, the conduit interconnecting the cylinders, and a reservoir connected to the master cylinder with hydraulic fluid before shipment of the assembly to a motor vehicle manufacturer for installation in a motor vehicle.
Whereas such prefilled and preassembled hydraulic apparatus contribute importantly to the efficient and inexpensive assembly of the motor vehicle, the master cylinders of these apparatuses are serviceable by the operator of the motor vehicle and such servicing often causes problems with respect to the functioning of the total system or with respect to leakage of hydraulic fluid from the system.